This invention relates to heat exchangers and, more particularly to motor vehicle heat exchangers or radiators and to a method of joining the members thereof.
Users of heat exchangers, particularly those used in automotive and truck engine-cooling radiators, are interested in achieving longer life in units which they purchase. Truck manufacturers are especially seeking a longer life radiator which can withstand the extra stress and abuse in a truck environment for significantly longer mileage.
The joints between the tubes and headers in heat exchangers in general, and automotive type radiators in particular, have received increased scrutiny in the goal to achieve longer service life. Typically, the tube-to-header joint had been a simple tin/lead soldered joint between a thin walled brass tube and a thicker header which has a collared hole to receive the tube. Because of expansion and contraction of the tube during the heating and cooling cycle present in operation, the joints between the tubes and the headers are placed in shear stress. Shock and vibration in the motor vehicle environment add to this stress. In addition, chemical reactions between the coolant and any solder which covers the header surface on its water side can cause corrosion products which can flake off and clog the radiator tubes, thereby leading to engine overheating.
Welding has been used to bond the tube-to-header joint in order to strengthen this portion of automotive radiators. However, current methods of welding do not provide a completely leak-free welded joint in every instance. To rectify this problem, it has been a practice to apply a coating of solder on the air side of all of the joints to seal any leaks in the weld area between the tube and header. This operation requires the placement of expensive perforated solder foil or other prefabricated forms of solder over the tubes on the radiator core prior to assembly of the header to the core. After welding the tubes to the header collars, the header is then dipped in flux and must be heated extensively to cause the solder foil to melt and form the sealant coating on the air side of the header. During this operation, solder flows through the usually numerous tube-to-header joint leaks and considerable solder appears on the water side of the header. This operation is energy intensive, time consuming, and often causes the header to warp due to the high temperatures in the heating operation. Also, the core must normally still be tested to determine the integrity of the tube-to-header joints. In spite of the air-side solder coating, there may often appear several joints which remain unsealed and leak coolant fluid. These joints must then be repaired, usually by hand methods such as using a torch and wire solder on the air side of the joint.
In an attempt to achieve leak-free tube-to-header joints, resizing of the radiator core tubes has been performed prior to and/or after attachment of the header. U.S. Pat. Nos. 4,744,505 and 4,858,686 disclose a method of resizing the ends of oval or non-circular header tubes to a circular shape, inserting the ends of the tubes into circular collar openings having a slightly larger diameter, and thereafter expanding the outermost portion of the tube ends to create a tight fit with the collar opening. However, even with such resizing, the subsequent welding of the joints still does not usually produce a leak-free joint and, consequently, solder is used to seal the welds. Furthermore, the many steps involved in resizing are time consuming and require positioning and handling of the heavy and unwieldy core. This adds to the cost of manufacture, and the difficulty of the operations contributes to wide variations in product quality.
Alternative methods of sealing the tube-to-header joints include applying flux to the liquid side of the bonded joint and then applying a coating of solder, for example, by dipping into a bath of molten solder or by the use of wave soldering techniques. These methods result in lower header temperatures and thereby minimize warping and the need for repair. However, the increased amount of solder left on the water side surface of the header creates a potential for the creation of solder corrosion products, also termed "solder bloom".
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide an improved tube-to-header joint for use in heat exchangers and, in particular, automotive type radiators.
It is another object of the present invention to provide a heat exchanger in which the tube-to-header heat exchanger joints are substantially leak-free.
It is a further object of the present invention to provide a tube-to-header heat exchanger joint which minimizes the need for post-production testing and repair for leaks.
It is another object of the present invention to provide a method of manufacturing a tube-to-header heat exchanger joint which minimizes complicated, time consuming and unwieldy handling of the heat exchanger.
It is a further object of the present invention to provide a method of manufacturing a tube-to-header heat exchanger joint which avoids resizing of tube ends after the core assembly is fitted to the header.
It is yet another object of the present invention to provide a tube-to-header heat exchanger joint which minimizes the amount of solder in contact with the heat exchanger liquid.
It is a further object of the present invention to provide a solder sealed tube-to-header heat exchanger joint which reduces the total amount of solder used, utilizes less expensive solder, and minimizes the effort needed to apply the solder during the production of the heat exchanger.
It is another object of the present invention to provide a welded solder sealed tube-to-header heat exchanger joint which can be readily manufactured by mass-production techniques and which results in an improved product.